As a method of purifying river water for use as service water and the like, a filtration method utilizing a porous hollow fiber membrane which improves safety of treated water and reduces the installation space is being widely used. A porous hollow fiber membrane is required to exhibit high blocking performance which can reliably remove bacteria (e.g., cryptosporidium) and components making water turbid, high water permeability for treating a large quantity of water, and high strength which enables long-term use under a wide range of operating conditions (e.g., chemical washing and operation under high operating pressure).
A concept of obtaining a porous multilayer hollow fiber membrane which exhibits high blocking performance and high water permeability by bonding a blocking layer having a small pore diameter and a strength support layer having a large pore diameter is disclosed in Patent Document 1, for example. Specifically, Patent Document 1 discloses a method in which a crystalline thermoplastic resin such as polyethylene is melt-extruded without adding a solvent, and a porous multilayer hollow fiber membrane is produced from the resulting hollow fiber extruded product using a stretch pore-forming method. The term “stretch pore-forming method” refers to a method in which a hollow fiber extruded product is stretched in the longitudinal direction at a high stretch ratio to cleave the lamellar crystal stack to obtain a porous membrane (see Non-patent Document 1). In Patent Document 1, crystalline thermoplastic resins which differ in melt index (MI) are melt-extruded from two circular nozzles disposed concentrically. This is because the method disclosed in Patent Document 1 utilizes the property that resins which differ in MI (i.e., differ in molecular weight) have different pore diameters upon stretch pore-forming. As a result, a porous two-layer hollow fiber membrane in which the outer layer and the inner layer differ in pore diameter is obtained. However, a porous multilayer hollow fiber membrane exhibiting high strength cannot be obtained by the method due to the following problems.
(1) The strength of the porous multilayer hollow fiber membrane in the stretch axis direction is increased by stretching at a high stretch ratio. However, bursting strength and compressive strength (i.e., strength in the direction perpendicular to the stretch axis) important for filtration tend to decrease.(2) In principle, the outer layer and the inner layer must differ in molecular weight or polymer type. However, required properties such as chemical resistance and mechanical strength differ depending on the molecular weight or polymer type. Therefore, when using a resin having low strength, the strength of the entire membrane decreases.
Therefore, a membrane exhibiting high strength cannot be obtained. Moreover, since a membrane obtained by this method has a structure in which the pore diameter in the longitudinal direction of the hollow fiber is larger than the pore diameter in the thickness direction, the membrane shows low bursting strength and low compressive strength.
Therefore, a porous multilayer hollow fiber membrane which exhibits high blocking performance, high water permeation rate, and high strength and a process for stably producing such a porous multilayer hollow fiber membrane have not yet been obtained.
A thermally induced phase separation method has been known as a method for producing a porous membrane. This method utilizes a thermoplastic resin and an organic liquid. The organic liquid serves as a latent solvent which does not dissolve the thermoplastic resin at room temperature, but dissolves the thermoplastic resin at a high temperature. In the thermally induced phase separation method, the thermoplastic resin and the organic liquid are mixed at a high temperature so that the thermoplastic resin is dissolved in the organic liquid. The mixture is then cooled to room temperature to induce phase separation. The organic liquid is then removed from the mixture to obtain a porous body. This method has the following advantages.
(a) A membrane can be easily produced using a polymer such as polyethylene for which an appropriate solvent which can dissolve the polymer at room temperature does not exist.
(b) Since the thermoplastic resin is dissolved at a high temperature and cooled to solidify and form a membrane. Therefore, particularly when the thermoplastic resin is a crystalline resin crystallization is promoted so that a high-strength membrane is easily obtained.
Therefore, the thermally induced phase separation method is widely used as a porous membrane production method (see Non-patent Documents 1 to 4, for example).    [Patent Document 1] JP-A-60-139815    [Patent Document 2] JP-A-3-215535    [Patent Document 3] JP-A-2002-56979    [Patent Document 4] JP-A-4-065505    [Non-patent Document 1] “Plastic and Functional Polymer Dictionary”, pp. 672 to 679 (Industrial Research Center of Japan, February, 2004)    [Non-patent Document 2] Hideto Matsuyama, “Production of Polymer Porous Membrane by Thermally Induced Phase Separation (TIPS) method”, Chemical Engineering, pp. 45 to 56 (Kagaku-Kogyo-Sha, June 1998)    [Non-patent Document 3] Akira Takizawa, “Membrane” pp. 404 to 406 (IPC, January 1992)    [Non-patent Document 4] D. R. Lloyd, et. al., Journal of Membrane Science, 64 (1991), pp. 1 to 11